The increasing demand for lightweight construction concepts in the automotive industry confers prominence on the use of sandwich panels which have a thermoplastic plastics-material layer between two thin metallic cover layers, so as to further increase the potential for weight saving in automotive construction while using sandwich panels. Sandwich panels are manufactured, for example, in the form of strips by laminating a metallic strip with a continuous thermoplastic plastics-material layer and with a further metallic strip, and said sandwich panels are trimmed to form panels. Sandwich panels may provide various mutually exclusive properties which open up new potentials for weight saving. In this way, by virtue of the plastics-material layer, sandwich panels are significantly lighter than solid metallic panels, while at the same time making available high strength values. Moreover, the sandwich panels are sound-damping and offer high rigidity. However, it is disadvantageous in the case of sandwich panels that they have an electrically isolating plastics-material layer which in the case of fusion welding methods causes problems in relation to the configuration of a flawless welded connection. By virtue of lacking suitability for welding, for example for resistance welding to other metallic components, sandwich panels are therefore often adhesively bonded or are joined together by mechanical means.
A method for joining a sandwich panel to a further metallic component is disclosed in the German publication DE 10 2011 109 708 A1, in which the intermediate layer in the connection region is melted and is displaced from the connection region such that a welded connection may be subsequently produced by establishing electrical contact between the component and the cover layers of the sandwich panel. It is proposed that heating of the joint regions is carried out by temperature-controllable electrodes or pressing elements. To this end the welding electrodes or the pressing elements are provided with heating elements, for example. The construction of the welding electrodes thus becomes relatively complex. Moreover, the speed of heating the thermoplastic plastic-material layer may be further increased such that shorter cycle times may be achieved.
Moreover, a method for resistance welding two composite panels using two welding electrodes is disclosed in patent document U.S. Pat. No. 4,650,951, said two welding electrodes being heated and thus heating and displacing the plastics-material layer interposed between the cover layers before actual welding commences.
Moreover, a method in which two current circuits are used in order to connect a sandwich panel to a further metallic component by resistance welding is disclosed in German patent application DE 10 2013 108 563 A1. The method here is divided into two steps, that is to say, on the one hand, into a preheating step in which the region to be welded is heated by way of a preheating current, the thermoplastic plastics-material being displaced from that region of the sandwich panel that is to be welded and, on the other hand, after preheating the sandwich panel is welded to the metallic component by applying a welding current. However, it has been established that in the course of this procedure short circuit may arise during contact between the cover layers while the preheating current is still being applied, on account of which spillings may arise in the sandwich panel which may lead to the formation of blisters or to delamination, respectively.